The present invention relates to an image-forming apparatus and specifically relates to a duplex image-forming apparatus having a function of forming images onto both sides of a transfer material (a recording sheet).
In an image-forming apparatus, such as a laser printer, etc., a printing operation is performed through the processes of exposing, developing, transferring and fixing. Concretely speaking, initially, a latent image to be printed is formed on a surface of a photoreceptor drum by irradiating a light beam (a laser beam), which is modulated on the basis of the image data, onto the photoreceptor drum, and then, the developing device develops the latent image with toner. The developed toner image is transferred onto the transfer material (the recording sheet), and then, the fixing device fixes the toner image onto the transfer material. As a result, the image is formed on the transfer material.
When fixing the toner image onto the transfer material, the transfer material tends to shrink, since the fixing heat dehydrates the transfer material during the fixing operation. Specifically, when performing the duplex printing, the shrinkage of the transfer material, generated during the fixing operation for the obverse surface of the transfer material, causes a misregistration error between obverse and reverse images, since the obverse image size becomes different from the reverse image size. To cope with this problem, it has been possible to adjust the magnification factor in a sheet feeding direction by changing the processing velocity between the obverse image forming operation and the reverse image forming operation.
It has been a problem, however, that the abovementioned method, for adjusting the magnification factor by changing the processing velocity, cannot be applied for adjusting the magnification factor in a width direction of the transfer material, though it is applicable for adjusting the magnification factor in a sheet feeding direction (a lateral direction of the transfer material). Especially in a color image-forming apparatus having a plurality of image-forming units each of which includes an exposure unit, a photoreceptor drum, a developing device, etc., even if an image-forming operation, performed in one of the plurality of image-forming units, is completed, sometimes, another image-forming operation, performed in another one of the plurality of image-forming units, still continues, due to gaps arranged between the exposure units (process gaps). Therefore, the change of the processing velocity, before the image-forming operations in all of the plurality of image-forming units are completed, would result in a color misregistration error. To avoid the color misregistration error, the processing velocity should be changed after the image-forming operations in all of the plurality of image-forming units are completed. Accordingly, it has been a problem that the total printing time is getting long, resulting in a reduction of the print productivity.